Many industrial processes spray compositions that contain viscous or solid polymeric components, such as coatings, adhesives, release agents, additives, gel coats, lubricants, and agricultural materials. To spray such materials, it has been common practice to use relatively large amounts of organic solvents. The solvents perform a variety of functions, such as to dissolve the polymers; to reduce viscosity for spraying; to provide a carrier medium for dispersions; and to give proper flow when the composition is sprayed onto a substrate, such as coalescence and leveling to form a smooth coherent coating film. However, the solvents released by the spray operation are a major source of air pollution. There are several patents which disclose new spray technology that can markedly reduce organic solvent emissions, by using environmentally acceptable supercritical fluids or subcritical compressed fluids, such as carbon dioxide or ethane, to replace the solvent fraction in solvent-borne compositions that is needed to obtain low spray viscosity: U.S. Pat. Nos. 4,923,720 and 5,108,799 disclose methods for using supercritical fluids for the spray application of coatings. U.S. Pat. No. 5,106,650 discloses methods for using supercritical carbon dioxide for the electrostatic spray application of coatings. U.S. Pat. No. 5,009,367 discloses methods for using supercritical fluids for obtaining wider airless sprays. U.S. Pat. No. 5,057,342 discloses methods for using supercritical fluids for obtaining feathered airless sprays. U.S. Pat. No. 4,882,107 discloses methods for using supercritical fluids to apply mold release agents, such as in the production of polyurethane foam. U.S. Pat. No. 5,066,522 discloses methods for using supercritical fluids to apply adhesive coatings. U.S. Pat. No. 5,171,613 discloses methods for maintaining a feathered airless spray pattern obtained using supercritical fluids by preventing cooling of the coating mixture before it is sprayed. U.S. Pat. No. 5,178,325 discloses methods for using supercritical fluids or subcritical compressed fluids for coating substrates in which the average spray velocity of a decompressive spray is reduced by using an elongated spray orifice.
Smith, in U.S. Pat. No. 4,582,731, issued Apr. 15, 1986; U.S. Pat. No. 4,734,227, issued Mar. 29, 1988; and U.S. Pat. No. 4,734,451, issued Mar. 29, 1988; discloses methods and apparatus for the deposition of thin films and the formation of powders through the molecular spray of solutes dissolved in supercritical fluid solvents, which may contain organic solvents. The concentration of said solutes are described as being quite dilute; on the order of 0.1 percent. In conventional applications, the solute concentration is normally 50 times or more greater than this level.
The molecular sprays disclosed in the Smith patents are defined as a spray "of individual molecules (atoms) or very small clusters of solute" which are in the order of about 30 Angstroms in diameter. These "droplets" are more than 10.sup.6 to 10.sup.9 less massive than the droplets formed in conventional methods that Smith refers to as "liquid spray" applications.
Supercritical fluids and subcritical compressed fluids have also been used with fuel sprays. Nielsen, in U.S. Pat. No. 5,170,727, issued Dec. 15, 1992, discloses methods for using supercritical fluids as diluents in combustion of liquid fuels and waste materials. Zhen, et al., in "The Atomization Behavior of Fuel Containing Dissolved Gas", a paper submitted to Atomization and Sprays, discloses the atomization of diesel fuel containing air or carbon dioxide dissolved under pressure at room temperature. Using this very low viscosity fuel, which contains no polymers, they studied the effect of pressure and nozzle length on droplet size, spray angle, and discharge coefficient for a diesel injector nozzle.
The conventional atomization mechanism of airless sprays is well known and is discussed rand illustrated by Dombroski, et al., in Chemical Engineering Science 18:203 (1963). The coating exits the orifice as a liquid film that becomes unstable from shear induced by its high velocity relative to the surrounding air. Waves grow in the liquid film, become unstable, and break up into liquid filaments that likewise become unstable and break up into droplets. Atomization occurs because cohesion and surface tension forces, which hold the liquid together, are overcome by shear and fluid inertia forces, which break it apart. This process is shown photographically for an actual paint in the brochure entitled "Cross-Cut.TM. Airless Spray Gun Nozzles", Nordson Corporation, Amherst, Ohio. Often the liquid film extends far enough from the orifice to be visible before atomizing into droplets. The sprays are generally angular in shape and have a relatively narrow fan width, that is, a fan width that is not much greater than the fan width rating of the spray tip being used. As used herein, the terms "liquid-film spray" and "liquid-film atomization" refer to a spray, spray fan, or spray pattern in which atomization occurs by this conventional mechanism.
In liquid-film atomization, however, the cohesion and surface tension forces are not entirely overcome and they can profoundly affect the spray, particularly for viscous coating compositions. Conventional airless spray techniques are known to produce coarse droplets and defective spray fans that limit their usefulness to applying low-quality coating films. Higher viscosity increases the viscous losses that occur within the spray orifice, which decreases the energy available for atomization. It also decreases shear intensity, which hinders the development of natural instabilities in the expanding liquid film. This delays atomization so that relatively large droplets are formed. The spray also characteristically forms a "tailing" or "fishtail" spray pattern. The surface tension and cohesive forces in the liquid film tend to gather liquid at the edges of the spray fan, which produces coarsely atomized jets of coating. Sometimes the jets separate from the spray and deposit separate bands of coating. At other times they thicken the edges so that more coating is deposited at the top and bottom than in the center of the spray. These deficiencies produce a nonuniform deposition pattern that makes it difficult to apply a uniform coating.
It is well known that liquid-film atomization can be improved if the liquid is made turbulent or agitated before it passes through the atomization orifice of the airless spray tip. Turbulent or agitated flow of the liquid as it exits the orifice promotes destabilization and disruption of the liquid film, which causes it to break up more readily into finer droplets and into a more uniform spray. For this reason, various types of turbulence promoters have been designed for use with conventional airless spray tips. Such turbulence promoters include various types of pre-orifices, diffusers, turbulence plates, restrictors, flow splitters/combiners, flow impingers, screens, baffles, vanes, and other inserts, devices, and flow networks known to those skilled in the art. Examples of turbulence promoters and the turbulent flow created in the spray tip are illustrated in the catalog entitled "Airless Nozzles and Accessories", Nordson Corporation, Amherst, Ohio. One such example is a turbulence plate that is inserted into the inlet of the airless spray tip, wherein it divides the flow into two high velocity streams that impinge against one another head on at a ninety-degree angle to the main flow direction. The turbulent discharge then flows through the atomization orifice. Another such example is a restrictor plate that contains a pre-orifice that is somewhat larger in diameter than the atomization orifice. It is so positioned behind the atomization orifice to create a liquid jet that discharges against the atomization orifice, thereby generating the desired turbulence.
Thus, to improve liquid-film atomization, conventional airless spray tips are designed to maximize velocity and turbulence as the liquid flows through the atomization orifice, for a given pressure drop across said orifice. In particular, in one important design standard, the flow path in the orifice is made very short in order to minimize flow resistance and reduction in turbulence. Furthermore, in addition to being used with turbulence promotion devices, conventional spray tips themselves are sometimes designed to promote turbulent or agitated flow of the liquid at the entrance to the atomization orifice. For example, the spray tip chamber that feeds liquid to the atomization orifice may be contoured such that liquid flows coming from opposite sides of the chamber converge and impact each other as they flow into the atomization orifice.
The most commonly used airless spray tip design is sometimes called a dome-style spray tip. Such a spray tip is disclosed in, for example, U.S. Pat. No. 4,097,000, issued Jun. 27, 1978. The spray tip of this prior art embodiment is described and illustrated in FIGS. 1a, 1b, and 1c of the aforementioned commonly assigned U.S. Pat. No. 5,178,325 and is not a part of the present invention. The figures illustrate fluid flow patterns within the feed passageway and through the orifice, which clearly demonstrate the aforementioned flow convergence and impaction. The orifice piece is typically made from a tungsten carbide casting. The feed passageway is a hollow dome-shaped chamber centered about the flow axis of the spray tip. This hollow dome is formed in the spray orifice body before final hardening. The hollow dome extends nearly the length of the spray orifice body to such a depth that the roof or wall has the desired thickness at the convergent end. The orifice is sometimes formed by cutting a v-shaped groove across the outside end of the dome such that the groove intersects and cuts into the hollow end of the dome. This creates an orifice with an elliptical, circular, or similarly shaped cross-section of very short length. The size (cross-sectional area) of the orifice is determined by the depth of the v-shaped groove; that is, a larger orifice passageway produces a larger flow rate.
The angle of the v-shaped groove regulates the fan width of the spray, as is known to those skilled in the art. A smaller angle (narrower groove) produces a wider fan. A larger angle (wider groove) produces a narrower fan.
Still another dome-style spray tip is disclosed, for example, in U.S. Pat. No. 3,556,411, issued Jan. 19, 1971, which is described and illustrated in FIGS. 2a and 2b of the aforementioned U.S. Pat. No. 5,178,325, and which represents embodiments which are not in accordance with the present invention. The first figure shows a spray nozzle assembly with a turbulence plate that discharges into the spray orifice body to promote turbulent flow through the spray orifice. The second figure shows an example of a spray orifice body that can be used in the spray nozzle assembly. It has a circular converging feed passageway that is intersected by a groove to form the orifice passageway.
Other types of dome-style spray tips, which have different mechanical features so as to produce a desired spray pattern, are disclosed in U.S. Pat. No. 3,647,147, issued Mar. 7, 1972; U.S. Pat. No. 3,659,787, issued May 2, 1972; U.S. Pat. No. 3,737,108, issued Jun. 5, 1973; U.S. Pat. No. 3,843,055, issued Oct. 22, 1974; and U.S. Pat. No. 3,754,710, issued Aug. 28, 1973.
A more recent airless spray tip design is called a Cross-Cut.TM. type spray tip, which is disclosed in U.S. Pat. No. 4,346,849, issued Aug. 31, 1982. It is illustrated in FIG. 3 of the aforementioned U.S. Pat. No. 5,178,325, which is also not in accordance with the present invention. It is made by cutting interpenetrating grooves, at right angles to each other, into opposite sides of a tungsten carbide spray orifice body. The groove on the pressurized or inlet side is wedge-shaped in cross-section. The groove on the unpressurized or exit side has a bottom portion that is trapezoidal in cross-section. The orifice passageway is formed by the interpenetration of the two grooves. This gives a rectangular-shaped spray orifice passageway that has very short flow path length. The width of the outer groove regulates the fan width of the spray.
From the foregoing prior art, it is clear that the design of airless spray nozzles teaches, in part, that it is desirable to maintain a very short orifice path length in order to maximize turbulent flow from the spray orifice.
All of these prior airless spray nozzle designs are directed to spraying conventional coating compositions. None of them uses supercritical fluids or subcritical compressed fluids as diluents to spray polymeric compositions.
The aforementioned Smith patents, which are directed to producing fine solid films and powders by using a "molecular" spray, disclose an apparatus which has an apparently elongated heated probe located within the sample collection chamber between the orifice and a transfer line coming from the heating oven. In particular, in the aforementioned Smith U.S. Pat. No. 4,734,227, it is taught that the process utilizes a fluid injection technique which calls for rapidly expanding the supercritical solution through a short orifice into the relatively lower pressure region. The text teaches away from use of long orifice designs because, as noted, more conventional nozzles or longer orifice designs would enhance solvent cluster formation, which is taught as being undesirable. Consequently, the function of the apparently elongated probe is to convey, within the sample collection chamber, the fluid between the transfer line, located outside of said chamber, and the prescribed short orifice.
As disclosed in the aforementioned patents, supercritical fluids or subcritical compressed fluids such as carbon dioxide or ethane are not only effective viscosity reducers, they can produce a new airless spray atomization mechanism, which can produce finer droplet size than by conventional airless spray methods and a feathered spray needed to apply high quality coatings. Without wishing to be bound by theory, the new type of atomization is believed to be produced by the dissolved carbon dioxide suddenly becoming exceedingly supersaturated as the spray mixture experiences a sudden and large drop in pressure in the spray orifice. This creates a very large driving force for gasification of the carbon dioxide. The carbon dioxide gas released from solution during depressurization expands in volume and produces an expansive force that overwhelms the cohesion, surface tension, and viscosity forces that oppose atomization and normally bind the fluid flow together.
A different atomization mechanism is evident because atomization appears to occur right at the spray orifice instead of away from it. Atomization is believed to be due not to break-up of a liquid film from shear with the surrounding air but, instead, to the expansive force of the carbon dioxide gas. Therefore, no liquid film is visible coming out of the nozzle.
Furthermore, because the spray is no longer bound by cohesion and surface tension forces, it typically leaves the nozzle at a much wider angle than normal airless sprays and produces a "feathered" spray with tapered edges like an air spray. This typically produces a rounded, parabolic-shaped spray fan, instead of the sharp angular fans typical of liquid-film sprays. The spray also typically has a much wider fan width than liquid-film sprays produced by the same spray tip. As used herein, the terms "decompressive spray" and "decompressive atomization" each refer to a spray, spray fan, or spray pattern that has the preceding characteristics.
Laser light scattering measurements and comparative spray tests show that this decompressive atomization can produce fine droplets that are in the same size range as air spray systems, instead of the relatively coarse droplets produced by liquid-film airless sprays. For a properly formulated coating composition, the droplet size range and distribution are ideal for minimizing orange peel and other surface defects commonly associated with spray application. This fine particle size provides ample surface area for the dissolved carbon dioxide to very rapidly diffuse from the droplets within a short distance from the spray nozzle. Therefore, the coating is essentially free of carbon dioxide before it is deposited onto the substrate.
Generally, the preferred upper limit of supercritical fluid addition is that which is capable of being miscible with the polymeric coating composition. This practical upper limit is generally recognizable when the admixture containing coating composition and supercritical fluid breaks down from one phase into two fluid phases.
The aforementioned related U.S. Pat. No. 5,178,325, issued Jan. 12, 1993, discloses an improved method for coating substrates by a decompressive spray produced by a supercritical fluid or a subcritical compressed fluid, whereby the average velocity of the decompressive spray is reduced, thereby minimizing the undesirable excessive momentum of the spray as well as the sideways deflection of the spray that occurs as it impacts a substrate. The spray nozzle is comprised of an elongated orifice passageway having a length sufficiently long in relation to the equivalent diameter so as to reduce the average spray velocity of the coating material in the decompressive spray.
Although the supercritical fluid spray methods have been successful, one difficult problem that is created is that the reformulated polymeric composition, which is called a concentrate, has increasingly higher viscosity as higher levels of solvent are removed to further reduce solvent emissions. Concentrate viscosities typically increase from a conventional viscosity of about 40 to 100 centipoise to about 800 to 5000 centipoise or more as more solvent is removed. Therefore, obtaining fine atomization becomes increasingly more difficult. This limits the amount of solvent that can be removed and hence the solids level that can be used in the concentrate. The poorer atomization gives poorer spray application quality, such as poorer coatings. Therefore a need clearly exists for methods by which atomization can be enhanced, when using supercritical fluids or subcritical compressed fluids to spray polymeric compositions, in order to reach higher solids levels and to obtain finer atomization which improves spray application quality.
One method of obtaining finer atomization from a decompressive spray of a polymeric composition is disclosed in U.S. Pat. No. 5,290,603. A spray of finely atomized liquid droplets of a polymeric composition is formed by using combinations of compressed fluid concentration, spray temperature, and spray pressure for which the liquid spray mixture passes through the liquid-liquid region of the phase diagram for the system during depressurization. Without wishing to be bound by theory, enhanced atomization is believed to occur because the dissolved compressed fluid, during depressurization in the spray orifice, nucleates to form a liquid compressed fluid phase before forming gaseous compressed fluid, instead of nucleating directly to a gaseous compressed fluid phase. Nucleation to a liquid compressed fluid phase is much more favorable energetically than to a gas compressed fluid phase. Therefore, nucleation should occur much more quickly during depressurization, that is, at higher pressure because much less supersaturation is required. Furthermore, a much higher concentration of nucleation sites should form in the decompressing fluid. These liquid nucleation sites of liquid compressed fluid readily vaporize to gaseous compressed fluid upon further depressurization, which creates an expansive force that is greater and more widely distributed in the decompressive spray than if the compressed fluid nucleated directly to fewer gas phase sites at a higher degree of supersaturation, that is, at lower pressure. This higher level and better distribution of expansive force is therefore more effective at overcoming the cohesion, surface tension, and viscosity forces that oppose atomization. Therefore, more intense atomization can occur. However, although the method has been effective in enhancing atomization of a decompressive spray, its usefulness has been found to be limited in application, because the spray conditions required are quite narrowly limited. Furthermore, it is not applicable to some polymer systems, because they lack the required phase behavior.
Therefore, a need clearly exists for a more widely applicable method to enhance atomization when using supercritical fluids or subcritical compressed fluids to spray polymeric compositions. Such a method would allow spray application at higher solids levels, to reduce solvent emissions, and with finer atomization, to improve the quality of the application, such as improved coatings. The method would also increase the range of conditions at which a decompressive spray is obtained for a given polymeric composition. This would increase the operating window for commercial use of the spray process, because the application results would become less sensitive to the precision with which the spray conditions could be maintained with commercial spray equipment.